Ok finally got somewhere
Seems that HTC Sync 64 drivers refuse to install correctly in W7.
Works fine in Vista.
So I've uploaded (hope HTC don't mind) the 64 bit drivers so you can manually place them in C:\Program Files (x86)\HTC\HTC Driver\Driver Files\Vista_x64
Grab them here:
http://the-asylum.com/files/HTC_Driver_64.zip
If you don't have the ADB driver installed anyway, or just change the incorrectly installed one, find that device in Windows (Devices and Printers -> Android Phone, double click -> Hardware) and do an Update driver pointed at that location. It should then come back as "My HTC".
Hope that helps.